In drilling wells for oil and gas exploration, understanding the structure and properties of the associated geological formation provides information to aid such exploration. A number of different measurements in a borehole can be performed to attain this understanding. Measurements may include using a neutron tool to make measurements to determine properties of a formation under investigation. Further, the usefulness, efficiency, and accuracy of traditional measurements may be related to the precision or quality of the techniques to attain and process data derived from such measurements. Techniques and apparatus to simplify measurements, to enhance processing of measured data, to enhance analysis of data from measurements to provide properties of a formation or borehole, or to provide combinations thereof can further aid in drilling operations.
Since its introduction several decades ago, pulsed neutron generators have been used by the oil and gas well logging industry for measuring certain properties of downhole formations. An example conventional pulsed neutron generator (NG) uses a Penning ion source neutron generator tube. For operation of such a NG tube, the Penning ion source section may consist of a cylindrical anode, a back cathode, and a front cathode with an aperture, and may require a constant magnetic field along the axis of symmetry that is provided by a permanent magnet. A high voltage pulse applied between the anode and cathode creates an arc discharge needed for initiating the ionization of a deuterium (D) gas in the ion source.
Penning discharge ion sources are notorious for the number of modes of operation and the shifting between them during operation. Penning ion sources are also known to produce mostly (˜90%) diatomic deuterium ions. Once a particular geometry is selected, the ion source operation primarily depends on three parameters: a) the deuterium gas equilibrium pressure, b) the anode-cathode voltage, and c) the magnetic field intensity B. The uniform magnetic field across the ion source required for operation determines the use of either a permanent magnet or a solenoid, which adds mass, volume, and complexity to the ion source. In addition, the materials used in the construction of the permanent magnets need to be such that their magnetic properties do not degrade when exposed to the high temperature (>400 C) required for out-gassing the NG tube. The use of a solenoid, instead of the permanent magnets, requires providing two electrical connections to the tube, as well as the associated circuitry to supply the solenoid's current. Penning discharge ion sources use a high voltage pulse in the range of 2-6 kV to initiate and maintain the discharge, which may further complicate the circuitry and may compromise the reliability of the NG tube, as well as the controlling instrument.